Based on the provided source material, there is no information regarding a city named Longgun in China. The source instead defines "long gun" as a category of firearms characterized by long barrels.
In the context of small arms, a long gun or longarm is typically designed to be held with both hands and braced against the shoulder, distinguishing it from a handgun which can be fired with one hand. In artillery, an artillery long gun is distinguished from field guns or howitzers.